An Effective Model for Crumpling in Two Dimensions?

We investigate the crumpling transition for a dynamically triangulated random surface embedded in two dimensions using an effective model in which the disordering effect of the $X$ variables on the correlations of the normals is replaced by a long-range ``antiferromagnetic'' term. We compare the results from a Monte Carlo simulation with those obtained for the standard action which retains the $X$'s and discuss the nature of the phase transition.Comment: 5 page

Fatigue following mild traumatic brain injury relates to visual processing and effort perception in the context of motor performance

Introduction: Following mild traumatic brain injury (mTBI), a substantial number of patients experience disabling fatigue for months after the initial injury. To date, the underlying mechanisms of fatigue remain unclear. Recently, it was shown that mTBI patients with persistent fatigue do not demonstrate increased performance fatigability (i.e., objective performance decline) during a sustained motor task. However, it is not known whether the neural activation required to sustain this performance is altered after mTBI. Methods: Blood oxygen level-dependent (BOLD) fMRI data were acquired from 19 mTBI patients (>3 months post-injury) and 19 control participants during two motor tasks. Force was recorded from the index finger abductors of both hands during submaximal contractions and a 2-minute maximal voluntary contraction (MVC) with the right hand. Voluntary muscle activation (i.e., CNS drive) was indexed during the sustained MVC using peripheral nerve stimulation. Fatigue was quantified using the Fatigue Severity Scale (FSS) and Modified Fatigue Impact Scale (MFIS). Questionnaire, task, and BOLD data were compared across groups, and linear regression was used to evaluate the relationship between BOLD-activity and fatigue in the mTBI group. Results: The mTBI patients reported significantly higher levels of fatigue (FSS: 5.3 vs. 2.6, p < 0.001). Both mTBI- and control groups demonstrated significant performance fatigability during the sustained MVC, but no significant differences in task performance or BOLD-activity were observed between groups. However, mTBI patients reporting higher FSS scores showed increased BOLD-activity in the bilateral visual cortices (mainly extrastriate) and the left midcingulate gyrus. Furthermore, across all participants mean voluntary muscle activation during the sustained MVC correlated with long lasting post-contraction BOLD-activation in the right insula and midcingulate cortex. Conclusion: The fMRI findings suggest that self-reported fatigue in mTBI may relate to visual processing and effort perception. Long lasting activation associated with high levels of CNS drive might be related to changes in cortical homeostasis in the context of high effort

Increased Ipsilateral M1 Activation after Incomplete Spinal Cord Injury Facilitates Motor Performance

Incomplete spinal cord injury (SCI) may result in muscle weakness and difficulties with force gradation. Although these impairments arise from the injury and subsequent changes at spinal levels, changes have also been demonstrated in the brain. Blood-oxygen-level dependent (BOLD) imaging was used to investigate these changes in brain activation in the context of unimanual contractions with the first dorsal interosseous muscle. BOLD- and force data were obtained in 19 individuals with SCI (AISA Impairment Scale [AIS] C/D, level C4-C8) and 24 able-bodied controls during maximal voluntary contractions (MVCs). To assess force modulation, participants performed 12 submaximal contractions with each hand (at 10, 30, 50, and 70% MVC) by matching their force level to a visual target. MVCs were weaker in the SCI group (both hands p < 0.001), but BOLD activation did not differ between SCI and control groups. For the submaximal contractions, force (as %MVC) was similar across groups. However, SCI participants showed increased activity of the ipsilateral motor cortex and contralateral cerebellum across all contractions, with no differential effect of force level. Activity of ipsilateral M1 was best explained by force of the target hand (vs. the non-target hand). In conclusion, the data suggest that after incomplete cervical SCI, individuals remain capable of producing maximal supraspinal drive and are able to modulate this drive adequately. Activity of the ipsilateral motor network appears to be task related, although it remains uncertain how this activity contributes to task performance and whether this effect could potentially be harnessed to improve motor functioning

Smooth Random Surfaces from Tight Immersions?

We investigate actions for dynamically triangulated random surfaces that consist of a gaussian or area term plus the {\it modulus} of the gaussian curvature and compare their behavior with both gaussian plus extrinsic curvature and ``Steiner'' actions.Comment: 7 page

Structural and functional features and significance of the physical linkage between ER and mitochondria

The role of mitochondria in cell metabolism and survival is controlled by calcium signals that are commonly transmitted at the close associations between mitochondria and endoplasmic reticulum (ER). However, the physical linkage of the ER–mitochondria interface and its relevance for cell function remains elusive. We show by electron tomography that ER and mitochondria are adjoined by tethers that are ∼10 nm at the smooth ER and ∼25 nm at the rough ER. Limited proteolysis separates ER from mitochondria, whereas expression of a short “synthetic linker” (<5 nm) leads to tightening of the associations. Although normal connections are necessary and sufficient for proper propagation of ER-derived calcium signals to the mitochondria, tightened connections, synthetic or naturally observed under apoptosis-inducing conditions, make mitochondria prone to Ca2+ overloading and ensuing permeability transition. These results reveal an unexpected dependence of cell function and survival on the maintenance of proper spacing between the ER and mitochondria

Folding transition of the triangular lattice in a discrete three--dimensional space

A vertex model introduced by M. Bowick, P. Di Francesco, O. Golinelli, and E. Guitter (cond-mat/9502063) describing the folding of the triangular lattice onto the face centered cubic lattice has been studied in the hexagon approximation of the cluster variation method. The model describes the behaviour of a polymerized membrane in a discrete three--dimensional space. We have introduced a curvature energy and a symmetry breaking field and studied the phase diagram of the resulting model. By varying the curvature energy parameter, a first-order transition has been found between a flat and a folded phase for any value of the symmetry breaking field.Comment: 11 pages, latex file, 2 postscript figure

Biogenic Macroporosity and lts Lattice Boltzmann Method Permeability in the Karst Biscayne Aquifer

We focus on two major problems in the study of paleokarst of the Biscayne aquifer in southeastem Florida: ( 1 ), current conceptual models of karst aquifers do not adequately characterize much of the eogenetic rnacropore system within the carbonate rocks of the Biscayne aquifer, and (2) standard laboratory core-analysis rnethods cannol be used lo accurately measure the permeability of highly macroporous carbonate core samples

The nature of the continuum limit in the 2D RP2RP^2 gauge model

The RP(2) gauge model is studied in 2D. We use Monte-Carlo renormalization techniques for blocking the mean spin-spin interaction, , and the mean gauge field plaquette, . The presence of the O(3) renormalized trajectory is verified and is consistent with the known three-loop beta-function. The first-order `vorticity' transition observed by Solomon et al. is confirmed, and the location of the terminating critical point is established. New scaling flows in (,) are observed associated with a large exponent kappa in the range 4~5. The scaling flows give rise to a strong cross-over effect between regions of high and low vorticity and are likely to induce an apparent signal for scaling in the cross-over region which we propose explains the scaling observed for RP(2), RP(3) and SO(4)-matrix models. The signal for this `pseudo' scaling will occur for the RP(2) spin model in the cross-over region which is the region in which computer simulations are done. We find that the RP(2) spin model is in the same universality class as the O(3) spin model but that it is likely to require a very large correlation length before the true scaling of this class sets in. We conjecture that the scaling flows are due either to the influence of a nearby new renormalized trajectory or to the ghost of the Kosterlitz-Thouless trajectory in the associated XY model.Comment: 29 pages, LATEX2e, 10 figures, uses styles[epsfig,latexsym

First-order transition of tethered membranes in 3d space

We study a model of phantom tethered membranes, embedded in three-dimensional space, by extensive Monte Carlo simulations. The membranes have hexagonal lattice structure where each monomer is interacting with six nearest-neighbors (NN). Tethering interaction between NN, as well as curvature penalty between NN triangles are taken into account. This model is new in the sense that NN interactions are taken into account by a truncated Lennard-Jones potential including both repulsive and attractive parts. The main result of our study is that the system undergoes a first-order crumpling transition from low temperature flat phase to high temperature crumpled phase, in contrast with early numerical results on models of tethered membranes.Comment: 5 pages, 6 figure